Majority of infant leukemias involve rearrangements in the MLL gene and are most often fatal. These leukemias are characterized by high expression of MEIS1 but the biologic roles of MEIS1 in leukemia are not defined. The long term goal is to improve the outcome for MLL-fusion gene leukemia by developing newer more effective therapies. The objective of this application is to determine the role of MEIS1 in MLL-fusion gene leukemia and in hematopoiesis with the aim of understanding the therapeutic utility and window of targeting MEIS1 as a therapy. The experimental model studied will be inducible MEIS1 knock-out mice and MLL-AF9 knock-in mice. The first specific aim is to determine the requirement of MEIS1 in the maintenance of MLL-AF9 leukemia and of normal hematopoiesis. The central hypothesis is that MEIS1 is critical for MLL-AF9 leukemia but dispensable for normal hematopoiesis. The experimental strategy will be inducible deletion of MEIS1 in the bone marrow of MLL-AF9 and otherwise normal mice. The MEIS1-deleted and MEIS1-replete cells will be compared in their ability to maintain and reconstitute normal and leukemic hematopoiesis. The second specific aim is to identify the molecular functions of MEIS1. We have generated preliminary data from shRNA knockdown and genetic deletion studies. The central hypothesis is that MEIS1 is required for the regulation of the cell cycle and of the hypoxia-response pathway. The role of the hypoxia-response regulator - Hif-1a and its regulation by MEIS1 will be determined using the inducible MEIS1 knockout models of leukemia. Finally, the role of MEIS1-binding proteins such PBX1 will also be studied in these experimental models. The proposed studies are innovative since they address the role of MEIS1 in established leukemia and in hematopoiesis using robust genetic models and address the molecular mechanisms engaged by MEIS1 in physiologic experimental models. The proposed research is significant because it will fundamentally enhance the existing knowledge of hematopoiesis and of MLL-fusion gene leukemia while also identifying targets for novel therapies.